UV-stimulated near-IR emission of Pr³⁺ in phosphate glass via twofold-coordinated Sn centers

The optical properties of Pr₂O₃ and SnO co-doped barium-phosphate glass prepared by the melt-quenching technique have been investigated. Optical absorption and X-ray photoelectron spectroscopy (XPS) are employed in the characterization of tin species. The prevalence of divalent tin is indicated by the XPS data in accord with a conspicuous absorption band detected around 287 nm ascribed to twofold-coordinated Sn centers (isoelectronic with Sn²⁺). Upon ultraviolet (UV) photoexcitation of the tin centers, near-infrared (IR) emission from Pr³⁺ ions is realized. An excitation spectrum acquired by monitoring Pr³⁺ emission from the ¹D₂ state at 1.03 μm revealed a broad band around 290 nm consistent with a SnPr donor-acceptor energy transfer channel. The data supports a mechanism starting with the singlet-to-singlet UV excitation of Sn centers, followed by the intersystem crossing populating their triplet states emitting in the visible. From these, energy transfer occurs to ³P₀, ³P₁, ¹I₆, and ³P₂ resonant states in Pr³⁺, from which the near-IR emitting states ¹D₂ and ¹G₄ are populated.